Apparatus for processing well logging signals including control means for controlling the signal resolution and the speed of response of the signal indicating means

ABSTRACT

In accordance with an illustrative embodiment of the present invention, logging measurements of radially shallow conductivity from an electrode logging tool, radially deep and medium conductivity from an induction logging tool, acoustic travel time from a sonic logging tool, as well as natural gamma-ray and spontaneous potential measurements are processed for recording as a function of borehole depth. More specifically, the shallow conductivity measurement signals are averaged and applied to a hybrid logarithmic circuit which operates to form a logarithm function thereof until the signal amplitude reaches a predetermined level at which time a linear function of the measurement signals is formed. The output signals from the hybrid logarithmic circuit are recorded with three separate degrees of resolution using only two galvanometers by multiplexing two signals on one galvanometer. A speed-up circuit is utilized to prevent the galvanometer retrace from appearing on the recording medium. The induction logging signals are processed by suitable computing circuits to sharpen their resolution and then applied to hybrid logarithmic and skin effect circuits which, in addition to forming the logarithmic and linear functions, provides a skin effect correction. The resulting processed induction logging measurements are recorded with differing degrees of resolution and the radially deep measurement signals are combined with acoustic travel time derived porosity measurements to produce indications of the resistivity of the formation fluids.

United States 1 1 July 31, 1973 Baker APPARATUS FOR PROCESSING WELLLOGGING SIGNALS INCLUDING CONTROL MEANS FOR CONTROLLING THE SIGNALRESOLUTION AND THE SPEED OF RESPONSE OF THE SIGNAL INDICATING MEANS [75]Inventor: John H. Baker, Houston, Tex.

[73] Assignee: Schlumberger Technology Corporation, New York, NY.

[22] Filed: Dec. 28, 1970 [21] Appl. No.: 105,681

Related U.S. Application Data [62] Division of Ser. No. 765,563, Oct. 7,1968, Pat. No.

Primary ExaminerGerard R. Strecker Att0rneyWm. R. Sherman, Stewart F.Moore, Jerry M. Presson, Edward M. Roney, Ernest R. Archambeau, David L.Moseley, Michael J. Berger and James C. Kesterson [5 7 ABSTRACT Inaccordance with an illustrative embodiment of the present invention,logging measurements of radially shallow conductivity from an electrodelogging tool, radially deep and medium conductivity from an inductionlogging tool, acoustic travel time from a sonic logging tool, as well asnatural gamma-ray and spontaneous potential measurements are processedfor recording as a function of borehole depth. More specifically, theshallow conductivity measurement signals are averaged and applied to ahybrid logarithmic circuit which operates to form a logarithm functionthereof until the signal amplitude reaches a predetermined level atwhich time a linear function of the measurement signals is formed. Theoutput signals from the hybrid logarithmic circuit are recorded withthree separate degrees of resolution using only two galvanometers bymultiplexing two signals on one galvanometer. A speed-up circuit isutilized to prevent the galvanometer retrace from appearing on therecording medium. The induction logging signals are processed bysuitable computing circuits to sharpen their resolution and then appliedto hybrid logarithmic and skin effect circuits which, in addition toforming the logarithmic and linear functions, provides a skin effectcorrection. The resulting processed induction logging measurements arerecorded with differing degrees of resolution and the radially deepmeasurement signals are combined with acoustic travel time derivedporosity measurements to produce indications of the resistivity of theformation fluids.

3 Claims, 4 Drawing Figures 1 July 31, 1973 United States Patent 1 BakerS RECORDER r APPARATUS-FOR PROCESSING WELL LOGGING SIGNALS INCLUDINGCONTROL MEANS FOR CONTROLLING THE SIGNAL RESOLUTION AND THE SPEED OFRESPONSE OF THE SIGNAL INDICATING MEANS This is a division ofapplication Ser. no. 765,563 filed on Oct. 7, 1968, now U.S. Pat. No.3,609,518.

This invention relates to methods and apparatus for processing welllogging signals derived from exploring devices which are passed througha borehole for investigating earth formations adjacent to the borehole.

In the investigation of earth formations adjacent a borehole, varioustypes of investigating devices are utilized for deriving information asto various formation characteristics and producing a log of suchinformation versus borehole depth. Among these devices are the socalledresistivity or conductivity devices which, as their names imply, measurethe resistivity or its reciprocal, conductivity, of earth formations.One way to produce such resistivity or conductivity measurements is toutilize either induction or coil type exploring devices wherein atransmitter inductively couples current into a formation, the magnitudeof this current being sensed by a nearby receiver to provide a measureof formation conductivity. Another form of resistivity or conductivityexploring devices utilize elecrodes for directly emitting current into aformation to obtain a measure of its resistivity or conductivity. Byappropriately arranging the electrodes and coils of these exploringdevices, the depth of investigation can be controlled.

An example of such resistivity or conductivity measuring devices havingdifferent depths of investigation can be found in copending applicationSer. No. 709,838 by Georges Attali, filed on Mar. 1, 1968 which is acontinuation of application Ser. No. 240,568, filed on Nov. 28, 1962. Inthis copending Attali application, an electrode array having arelatively shallow depth of investigation is combined with two inductioncoil arrays which have medium and deep depth of investigation. As setforth in the Attali application, the induction logging signals aredesirably corrected for the so-called skin effect" phenomenon to producemore accurate measurements. By producing logs derived from exploringdevices having different depths of investigation, the depth of invasionof the drilling'mud contained in the borehole into the formation can bereadily determined since the resistivity of the drilling mud isgenerally different from the resistivity of the connate formationfluids.

As stated in this copending Attali application, it is desirable torecord these conductivity or resistivity well logging signals inlogarithmic form. Among the advantages of such a logarithmicpresentation is that measurements having a large dynamic range can berecorded on one film or track, yet good resolution is attainable in themeasurement ranges of most interest. Another advantage of such alogarithmic presentation is that multiplication or division by aconstant can readily be achieved by merely sliding one curve relative toanother, as by an overlay technique.

Although a logarithmic function is essentially a compressed scalefunction, i.e., the resolution at high signal levels is less than theresolution at low signal levels, it has been found that the range ofamplitudes of well logging signals is sometimes so great that even withlogarithmic recording of such well logging signals on standard sizefilms, the recorded traces will sometimes go off scale due to the largedynamic range of most well logging measurements. As can be expected, alog analyst may become disturbed when there are no recorded results onlarge portions of a log. For one thing, there may be a suspicion thatthe investigating equipment has broken down.

Generally, it is an object of the present invention to provide new andimproved methods and apparatus for processing well logging signals.

In accordance with one form of the present invention, methods andapparatus for processing well logging signals comprises utilizing aconverting means for converting an input well logging signal to anoutput signal which varies substantially as a logarithmic function ofthe input well logging signal. The output signal can then be monitoredto enable the converting means to change from a logarithmic transferfunction to a linear transfer function upon the output signal attaininga given threshold level. Also, the logarithmic output signal from theconverting means can be inverted in polarity and applied to an antilogcircuit to produce a linear inverse function of the input well loggingsignal, thus avoiding the necessity of using more complicated types ofreciprocators.

If the well logging signals are derived from the socalled inductionlogging type of exploring device, skin effect correction can beimplemented by adding a predetermined fraction of the input well loggingsignal to the logarithmic function signal. Moreover, all of theabove-discussed operations can be carried out simultaneously.

In accordance with another form of the present invention, a well loggingsignal having a large dynamic range can be linearly recorded byutilizing a plurality of scale ranges, or resolutions, i.e., recordingthe same signal multiplied by a plurality of different gain factors. Toeconomize on the number of required galvanometers, at least two signalshaving different resolutions can be multiplexed. To this end, theamplitude of the signal to be recorded can be monitored to enable theselection of the desired resolution. So that the galvanometer will notproduce a visible trace when switching between the differing resolutionsignals, a large amplitude signal can be supplied to the galvanometer atthe time of switching to speed up the retrace velocity, i.e., thevelocity with which the galvanometer trace moves across the recordmedium from one position to another.

For a better understanding of the present invention, together with otherand further objects thereof, reference is had to the followingdescription taken in connection with the accompanying drawings, thescope of the invention being pointed out in the appended claims.

Referring to the drawings:

FIG. 1 shows an investigating apparatus having multiple exploringdevices in a borehole along with a schematic representation of a systemfor processing the well logging signals derived from such apparatus inaccordance with the present invention;

FIGS. 2 and 3 show the transfer characteristics of various portions ofthe circuitry of FIG. 1; and

FIG. 4 shows a typical example of logs that could be expected from theFIG. 1 system.

Now referring to FIG. 1, there is shown an investigating apparatus 10lowered in a borehole 11 on the end of an armored multiconductor cable12 for investigating earth formations 13. The lower feet or so of thecable 12 is covered with a suitable insulating material 14 and also hasa pair of electrodes B and N mounted thereon. The investigatingapparatus includes deep and medium investigation induction loggingdevices, a shallow investigation electrode device, a spontaneouspotential measuring device, an acoustic travel time measuring device anda natural gamma-ray measuring device. The induction and electrodelogging exploring devices as well as the spontaneous potential exploringdevice are shown in detail in the copending Attali application Ser. No.709,838. An example of a sonic exploring device can be found in U. S.Pat. No. 3,231,041 granted to F. P. Kokesh on Jan. 25, 1966. The measurepoints of each of the exploring devices are represented by X's on theinvestigating apparatus 15 with the gamma-ray measure point M being thehighest measure point. The lowest measure point is the measure point forthe shallow electrical log and spontaneous potential, designatedrespectively M and M The medium and deep induction log measure pointsandjhe sonic measure pointf desi g nated M TfKfiw, and M respectivelyare intermediate of the gamma ray and shallow electrical logmeasurements.

The signals derived from each of the exploring devices are transmittedby way of various conductors within the armored multiconductor cable 12to suitable signal processing circuits 16 at the surface of the earth.These signal processing circuits l6 perform various signal processingfunctions such as referencing the various signals to ground, gaincontrol, and, in general preparing the signals for application to thevarious circuits at the surface of the earth.

Considering first the conductivity signal o derived from the shallowelectrical logging device, this signal is applied to suitable memorizerand averaging circuits 17. These circuits act to memorize theconductivity signal over a given depth interval and read out certainones of the memorized signals to an averaging circuit. This operationtends to decrease the resolution of the shallow electrical logmeasurement so as to be matched with the induction logging measurements.The memorizer portion of circuit 17 is driven as a function of boreholedepth by a shaft 22 coupled to a wheel 21 which engages the cable 14.The memorizer and averaging circuits 17 then generate an averagedconductivity signal o' and a second unaveraged signal (f The primedesignations on all of the signals of FIG. 1 indicate that these signalsare all referenced to the same depth level. Thus, the reading out of thesignals from the memorizer portion of circuits 17 is se lected such thattwo output signals therefrom will both be at the same depth level. Thememorizer and averaging circuits 17 are shown in greater detail incopending application Ser. No. 749,158 by H. G. Doll, filed on July 31,1968.

Considering first the averaged signals (r' this signal is supplied to aprocessing circuit 18. Considering the processing circuit 18 in greaterdetail, the 0'40 4mg, signal is fed to a logarithmic amplifier 19 via asuitable input resistor 20. The logarithmic amplifier 19 could, forexample, take the form of the logarithmic circuit shown in the copendingAttali application Ser. No. 709,838 or, for that matter, any form oflogarithmic amplifier. The output signal from the logarithmic amplifier19 is thus an inverted logarithmic function of the applied input signal,i.e., -log0' This logarithmic signal is then applied to a galvanometertype recorder 23 whose record medium is driven as a function of depth bythe shaft 22 and to an antilog circuit 24 for further processing (to bediscussed later). Inside the recorder 23, this signal is used toenergize a solenoid 39a for causing a galvanometer mirror 40a to assumean angular position representative of the current through solenoid 39a.A light source 41 emits a beam of light which is reflected ofi of themirror onto track B of a record medium 34. The record medium 34 isdriven by the shaft 22 as a function of depth to thereby make a log oflog rr' As discussed earlier, one of the advantages of con verting welllogging signals having a large dynamic range to logarithmic functionsthereof is that the scale becomes automatically compressed and goodresolution is maintained throughout the lower ranges of primaryinterest. However, as disussed earlier, even when utilizing such alogarithmic presentation, it is not entirely uncommon for themeasurements to go off scale. To circumvent this, a gated linearfeedback circuit 25 detects when the output signal from the logarithmicamplifier l9 exceeds a predetermined threshold level, which level isnear the edge of the recording scale, i.e., nearly off scale, and causesa negative feedback action through a resistor 26 such that the outputsignal from the logarithmic amplifier 19 will vary in a relatively lowresolution linear manner after this threshold level has been attained.

More specifically, the negative output signal from logarithmic amplifierl9 (i.e., negative if ais positive) is inverted in polarity by anamplifier 26. The positive output signal from amplifier 26 is thencompared with a negative reference voltage by way of a pair of resistors27 and 28 and the difference therebetween applied to the input of anoperational amplifier 29. When the current through resistor 27 from theamplifier 26 is less than the current through resistor 28 from thenegative reference voltage source, the output of the operationalamplifier 29 will be positive thus causing the output current fromamplifier 29 to be fed back to its input by way of a diode 30. Aresistor 26 is connected to the anode of a diode 31 whose cathode isconnected to the output of amplifier 29 such that when the amplifier 29output is positive, the diode 31 will be back-biased thus preventingcurrent flow through the resistor 26.

Now, when the current through resistor 27 exceeds the current throughresistor 28, the output of the amplifier 29 will become negative thusallowing current to flow through the resistor 26 and a resistor 33 whichis connected from the input of amplifier 29 to the anode of diode 31.The feedback current then acts to maintain the input of logarithmicamplifier l9 and thus the output thereof at a substantially constantvalue. How constant this value remains depends on the ratio of thevalues of resistors 27 and 33. Thus, by appropriately selecting thevalues of these two resistors, the overall closed loop gain of thesystem can be set at a desired level so that the output of logarithmicamplifier 19 will change linearly with the applied input signal 0" Thisentire circuit just described makes up a hybrid logarithmic circuit,represented by the dashed line box Referring to FIG. 2, there is shownthe transfer curve of the circuit just described. The input voltage e(output voltage from circuits 17) is shown on the horizontal axis andthe output voltage e from logarithmic amplifier 19 is shown on thevertical axis. The curve 35 represents the logarithmic transfer functionproduced by the logarithmic amplifier 19. It can be seen from the curve35 of FIG. 2 that the output voltage will become exceedingly high forvery small values of input voltage, i.e., conductivity. Thus, if a'becomes low enough, the log being traced on the record medium would gooff scale. After the output voltage e reaches the threshold level,designated e, in FIG. 2, the transfer function changes to the linearfunction represented by the line 36. Thus the overall circuit transferfunction is represented by the solid line portion of lines 35 and 36 inFIG. 2.

It should be noted from FIG. 2 that negative conductivity signals can bereadily recorded when using this linear transfer function, which wouldnot be possible with just the logarithmic transfer function. (Eventhough it is theoretically impossible to have negative conductivitysignals, it nonetheless happens from time to time that negativeconductivity signals are produced by the downhole exploring device forone reason or another and it is advantageous for the well log analyst tobe able to determine at a glance that negative measurements are beingmade.)

It is to be understood that the slope of the line 36 as well as thethreshold level e, can be set at any given value beside the onesdepicted in FIG. 2. Thus, it would be possible, if desired, to lower thethreshold level e, from the point shown in FIG. 2 and adjust the slopeof line 36 such that the transition from logarithmic to linear transferfunctions will be relatively smooth.

As stated earlier, it is many times desirable to convert a measuredconductivity signal to a resistivity signal. Prior to this time, such anoperation would require a more complicated reciprocator. However, byutilizing the apparatus of the present invention, this reciprocatingoperation can be performed in a relatively simple and accurate manner bytaking the antilog of a negative logarithmic signal. Thus, in FIG. 1,the output signal from logarithmic amplifier 19, which is proportionalto log a' is applied to the antilog circuit 24 which produces a linearresistivity signal, designated R This resistivity signal is thensupplied to the recorder 23 by way of a resistor 38 whose value isappropriately selected to produce a scale multiplication factor of l.

Inside the recorder 23, the current through resistor 38 is supplied to agalvanometer winding 39 for causing a rotatable mirror 40 to assume anangular position representative of the magnitude of the current throughcoil 39. The light source 41 emits a light beam which is reflected offof the mirror 40 onto the recording medium (e.g., film) 42 to therebymake a log of R' ,,,,,,,X l on track A of the record medium 42.

As discussed earlier, when recording well logging signalsin a linearfashion, the large dynamic range of most well logging signals may wellrequire more than one recording of the same measurement, but usingdifferent scales. In fact, to achieve reasonably good resolution at allamplitude levels of the shallow resistivity measurement R' it has beenfound that three logs having different scales should be recorded.Normally, this would require three separate galvanometers. However, inthe FIG. 1 system, three logs are recorded with only two galvanometers.To accomplish this, the output signal from the antilog circuit 24 issupplied to a suitable voltage comparator 44 which energizes a relaysolenoid 45. In the normally closed position of the relay switch 45b,the resistivity signal R' is supplied to a galvanometer coil 46 by wayof a resistor 47. The value of the resistor 47 is selected such as toprovide a multiplication factor 5. When the resistivity signal R'exceeds the threshold level of voltage comparator 44, the relay switch45b switches to its normally open posi tion so as to connect anotherresistor 48 to the galvanometer coil The resistance value of resistor 48is selected such as to multiply the resistivity signal by a factor of0.1. To prevent the comparator 44 from continuously changing state whenR' is at or near the threshold level, some hysteresis is built into thecomparator so that the threshold level going up (increasing voltage) isdifferent from the level coming down (decreasing voltage).

The galvanometer coil 46 causes a galvanometer mirror 49 to reflect abeam of light from the light source 41 onto the record medium 42 on thesame track that the R' 1 log is being recorded. By this means, one ofthe R ,,,,,,,,X 0.1 or R' ,X 5 signals will be recorded, depending onthe value of R mrmm, through the use of only one galvanometer. Thus, alinear presentation of a measurement having a large dynamic range can bemade while at the same time maintaining a good degree of resolution overthe entire dynamic range of the measurement.

Whenever the relay 45 switches, the galvanometer mirror 49 will have toretrace, i.e., move from one end of the record medium to the other.Unless this movement is very fast, the retrace will undesirably show upon the record medium as part of the log. To prevent this from happening,the relay solenoid 45a causes another relay switch 45c to switch backand forth between ground and a positive DC voltage each time thecomparator 44 output changes state. The resulting square wave isdifferentiated by a capacitor 53 and resistor 54 to produce a sharppositive or negative pulse to speed up the mirrorv49 movement. Theconnections of relay switches 45b and 45c are made such that a positivepulse will be produced whenever the relay switch 45b switches from the5x scale to the 0.1x scale and a negative pulse will be produced for theopposite case.

Due to the rapid movement of the galvanometer mirror 49, it will swingrapidly to an extreme angular position and then return exponentially tothe correct measurement value. This would tend to produce an undesirableovershoot on the record medium. To prevent this overshoot from showingup on the record medium 34, a pair of opaque optical masks 50 and 51 arelocated between the galvanometer mirror 49 and the record medium 34. Themask 50 is positioned such as to intercept the light beam whenever itwould impinge on the record medium at a point lower than 10 percent offull scale. The mask 51 prevents the mirror 49 light beam from goingbeyond the edge of the record medium on the full scale side thereof andthe mirror 40 light beam from going below 10 percent of full scaledeflection. Additionally, a third optically opaque mask 52 is positionedsuch as to intercept light from the galvanometer mirror 40 whenever thisbeam of light would go beyond full scale deflection on the record trackA. The stops 51 and 52 prevent the traces being recorded on track A frominterfering with the logarithmic curve being recorded on track B.Suitable stop means (not shown) are arranged relative to thegalvanometer mirrors 40 and 49 to prevent the reflected light from thesemirrors from going beyond the dimensions of the opaque masks.

Along with recording the averaged values of resistiv ity derived fromthe shallow investigation device, it would also be beneficial to recordan unaveraged shallow investigation resistivity measurement to enablethe detection of thin formation beds, such as the highly resistivelignite beds commonly found in the Gulf Coast. To this end, theunaveraged conductivity signal a' from the memorizer and averagingcircuits 17 is applied to another hybrid logarithmic circuit 55, whichis the same in construction as the above discussed hybrid logarithmiccircuit 18a. The output signal from this circuit 55 is thus proportionalto log a and is supplied to the recorder 23 for recordation as afunction of depth. Additionally, the output signal from the hybridlogarithmic circuit 55 is applied to an antilog circuit 56 whichsupplies a signal proportional to R' to the recorder 23. (For brevity ofthe drawings only the three gulvunometcrs already discussed will beshown in i"l(i. l.)

The deep induction log conductivity signal c from the signal processingcircuits I6 is applied to a memorizer and computing circuit 58 whichoperates to store the induction logging signals over a given depthinterval (through the action of shaft 22) for subsequent readout to acomputing circuit to produce a signal having better vertical (depthwise)resolution than the raw input signal. Apparatus for performing thisfunction can be found in U. S. Pat. No. 3,166,709 granted to H. G. Dollon Jan. 19, 1965. This computed output signal is supplied to a signalprocessing circuit 59 which operates to convert the computed signal to ahybrid logarithmic function of the original input signal in the samemanner that the shallow investigation conductivity signal a",;, wasconverted by the hybrid logarithmic circuit 18a. The processing circuit59 additionally corrects the induction log derived conductivity signalfor so-called skin effect. As is well known in the well logging field,the resistivity or conductivity values derived from an induction loggingtype exploring device will tend to have an error component due to skineffect at high conductivity values. It has been found that the trueformation conductivity 0-,, as measured by an induction logging device,can be expressed in terms of the apparent or measured value ofconductivity 0-,, as:

(l) where K and n are constants representative of the coil geometry andoperating frequency and e is an error term. (Desirably n is adjustedduring calibration to make 6 as small as possible.)

To bring about these results, the computed conductivity signal afi fromcircuit 58 is applied to a logarithmic and skin effect circuit 57.inside the circuit 57, this signal is supplied to a logarithmicamplifier 60 by way of an input resistor 61. This signal is, in effect,a computed version of the apparent deep conductivity. The output signalfrom the logarithmic amplifier 60 is thus porportional to -log a",, andis fed to the negative or polarity inverting input of an operationalamplifier 62 by way of a coupling resistor 63. The computed conductivitysignal d is supplied to the positive input of the operational amplifier62 by way of a resistor 64. Thus, the output from operational amplitier62 will be proportional to log 0',, K U'" The values of resistors 63 and64 are se lected to provide the constant K of equation (I).

A gated linear feedback circuit 65 then monitors the output of theoperational amplifier 62 in the same manner and for the same reasons asthe gated linear feedback circuit 25 did in the hybrid logarithmiccircuit 18a. This output signal from operational amplifier 62 is thusproportional to the logarithm of the true conductivity 0" as measured bythe deep investigation induction logging device, i.e., log 0",

If desired, the conductivity signal derived from the deep investigationinduction logging device can also be recorded in linear form. To thisend, log 0' can be supplied to an antilog circuit 66 which produces anoutput signal proportional to the true deep conductivity 0', which canbe recorded by recorder 23 if desired. The output signal fromoperational amplifier 22 can also be inverted by an inverting amplifier67 for application to an anti-log circuit 68. The output signal from theantilog circuit 68 will thus be proportional to the true deepresistivity R' Since this signal is in linear form, it is desirablyrecorded on one track with several different resolutions (in this casetwo separate scales or gain factors are used as determined by a pair ofresistors 69 and 70.)

Referring now to FIG. 3, there is shown a plot of the transfercharacteristics of the logarithmic and skin effect circuit 57. The inputvoltage e to circuit 57 is represented on the horizontal axis and theoutput voltage e therefrom is represented on the vertical axis. Thehybrid logarithmic transfer function is represented by the solid linecurve 77 and is essentially the same as the solid line curve in FIG. 2.The skin effect correction is represented by the solid line curve 78 andthe combination of the hybrid logarithmic function curve and the skineffect correction curve is represented by the dashed line curve 79.Thus, the dashed line curve 79 corresponds to the overall transferfunction of the lagarithmic and skin effect circuit 57.

The apparent conductivity signal derived from the medium investigationinduction logging device is supplied to memorizer and computing circuitswhich operate to perform essentially the same functions as the memorizerand computing circuits 58 for the deep investigation induction loggingsignal. The output signal from the memorizer and computing circuits 75is thus a computed version of the apparent medium investigationconductivity signal, designated o",, This signal is applied to alogarithmic and skin effect circuit 76 which is arranged in a similarmanner to the earlier discussed logarithmic and skin effect circuit 57except that the constants would be somewhat different. The output signalfrom the logarithmic and skin effect circuit 76 is thus proportional tothe logarithm of the true medium investigation resistivity and isdesignated log R',,,,. This logarithmic signal is then recorded byrecorder 23 as a function of borehole depth.

The spontaneous potential and natural gamma ray measurements from signalprocessing circuits 16 are supplied to a memorizer 80 which is alsodriven as a function of borehole depth by the shaft 22. The memorized SPand gamma ray measurements, designated SP and GR respectively, are thensupplied to the recorder 23 for recording as a function of depth. TheseSP and GR measurements can be utilized to indicate shale formation beds,as well as for depth control purposes, i.e.,

the SP or GR measurements can be correlated with an SP or GR measurementtaken during a different run in the borehole.

The sonic At measurement from the signal processing circuits I6 issupplied to a memorizer 8l which is also driven by tli'esh'ifi i'z. Thedepth earraasd Ar measurement from memorizer 81, designated At issupplied to a porosity computer 82 which computes the sonic derivedporosity (1),. in accordance with wylliek time average formula in amanner well known in the art. This porosity signal is supplied to alogarithmic amplifier 83 which produces an output signal proportional tothe logarithm of the sonic derived porosity, i.e., log

As set forth in U. S. Pat. No. 3,180,141 granted to R. P. Alger on Apr.27, I965, a combination ofa sonic derived porosity measurement with atrue formation resistivity R, measurement obtained from a relativelydeep investigation exploring device will give the apparent waterresistivity R in accordance with the expression:

(2) where a and m are exponents dependent upon the rock structure. (m 1may be used for most formations, a 0.81 may be used for most rockstructure, and a l.0 for most carbonates.) As set forth in this Algerpatent, a log of the apparent water resistivity R is beneficial inindicating various formation conditions.

Referring back to FIG. 1, to solve equation (2), the output fromlogarithmic amplifier 83 is supplied to the noninverting input of anoperational amplifier 84 by way of a variable resistor 85. Additionally,the output signal from the operational amplifier 62 of the processingcircuits 59 supplies a signal proportional to log to the inverting inputof the operational amplifier 84 by way of a resistor 86. The outputsignal from operational amplifier is thus proportional to log R +m log(11', where m is determined by the value of resistor 85. This outputsignal from operational amplifier 84 is then supplied to an antilogcircuit 87 which converts the logarithmic signals to a linear signalproportional to R' da', The multiplying factor a of equation (2) isproduced by a suitable potentiometer 88 on the output of the antilogcircuit 87 so as to produce a signal in accordance with equation (2) forrecording by recorder 23 as a function of borehole depth.

Now referring to FIG. 4, there is shown a typcial example of some of therecorded logs that can be produced by the apparatus of FIG. 1. Thespontaneous potential is recorded in track 1 and the logarithmic deep,medium, and shallow investigation resistivity signals are recorded intrack 2. As set forth in the copending Attali application, these logs intracks 1 and 2 can be utilized in the determiation of various formationconditions, such as water and hydrocarbon saturation. The averagedshallow investigation measurements are also recorded in linear form ontrack 3. As stated earlier, such a linear presentation enables easycorrelation of these logs with earlier derived linear type logs. Thesolid line curve in track 3 represents the 1x scale and the dotted linecurve represents the 5x scale. The dashdot line curve represents the0.1x scale.

In track 3, it can be seen that at least one of the 1x,

of the scale to prevent the galvanometer overshoot from showing up, this10 percent line being designated in FIG. 4. When the resistivity valuesare relative low, the 5x curve is the most informative or useful curveand when the resistivity values are very high, the 0.1x curve is themost beneficial curve. Thus, with the presentation depicted in track 3of FIG. 4, a linear resistivity presentation can be made and yet a highdegree of resolution can be maintained for substantially all resistivityvalues. Moreover, the three curve presentation of track 3 has been madewith the use of only two galvanometers and without unsightly tracesarising out of the galvanometer switching operation.

The unaveraged shallow resistivity log is shown recorded in track 4. Ascan be seen by comparing the averaged cruves of track 3 and thelogarithmic unaveraged R Hrs curve of track 2 with the track 4 unaveraged curve, this track 4 curve has much greater detail and can thus beused to readily spot thin formation beds having different resistivitiesthan the surrounding formations.

The apparent water resistivity R is shown recorded in track 5 and can beutilized in conjunction with the curves recorded in tracks 1 and 2 toprovide a great amount of information as to various formationconditions, as for example, distinguishing between water and hydrocarbonformation zones.

While there have been described what are at present considered to bepreferred embodiments of this invention, it will be obvious to thoseskilled in the art that varous changes and modifications may be made'therein without departing from the invention, and it is, therefore,intended to cover all such changes and modifications as fall within thetrue spirit and scope of the invention.

What is claimed is:

1. Apparatus for processing well logging signals derived from anexploring means in a borehole, comprismg:

means adapted to receive a well logging signal and apply a givenfunction of saie well logging signal to an indicating means;

means responsive to the amplitude of said well logging signal forproducing a control signal representative of whether said amplitudeexceeds a given threshold level; gain switching means responsive to saidcontrol signal for controlling the resolution of said given functionsignal which is applied to an indicating means;

means for producing a relatively high amplitude signal in response tochanges in said control signal; and

means for combining said high amplitude signal with said given functionsignal so that the combined signal will have a large change in amplitudelevel whenever said gain switching means is energized.

2. Apparatus for processing well logging signals derived from anexploring means in a borehole, comprismg:

means adapted to receive a well logging signal and apply a givenfunction of said well logging signal to a galvanometric indicating meansto produce a record thereof;

means responsive to the amplitude of said well logging signal forproducing a control signal representative of whether said amplitudeexceeds a given threshold level;

gain switching means responsive to said control signal for controllingthe resolution of said given function signal which is applied to anindicating means; means for producing a relatively high amplitude signalin response to changes in said control signal; and means for combiningsaid high amplitude: signal with said given function signal so that thegalvanometric indicating means will rapidly deflect from one position toanother whenever the resolution is changed. 3. A method of processingwell logging signals derived from an exploring means in a borehole,comprisreceiving a well logging signal representative of a formationcharacteristic and applying a given function of said well logging signalto an indicating means; producing at least two signals representative ofat whenever said control signal changes state.

i i i i i

1. Apparatus for processing well logging signals derived from anexploring means in a borehole, comprising: means adapted to receive awell logging signal and apply a given function of saie well loggingsignal to an indicating means; means responsive to the amplitude of saidwell logging signal for producing a control signal representative ofwhether said amplitude exceeds a given threshold level; gain switchingmeans responsive to said control signal for controlling the resolutionof said given function signal which is applied to an indicating means;means for producing a relatively high amplitude signal in response tochanges in said control signal; and means for combining said highamplitude signal with said given function signal so that the combinedsignal will have a large change in amplitude level whenever said gainswitching means is energized.
 2. Apparatus for processing well loggingsignals derived from an exploring means in a borehole, comprising: meansadapted to receive a well logging signal and apply a given function ofsaid well logging signal to a galvanometric indicating means to producea record thereof; means responsive to the amplitude of said well loggingsignal for producing a control signal representative of whether saidamplitude exceeds a given threshold level; gain switching meansresponsive to said control signal for controlling the resolution of saidgiven function signal which is applied to an indicating means; means forproducing a relatively high amplitude signal in response to changes insaid control signal; and means for combining said high amplitude signalwith said given function signal so that the galvanometric indicatingmeans will rapidly deflect from one position to another whenever theresolution is changed.
 3. A method of processing well logging signalsderived from an exploring means in a borehole, comprising: receiving awell logging signal representative of a formation characteristic andapplying a given function of said well logging signal to an indicatingmeans; producing at least two signals representative of at least twoseparate resolutions of said given function signal; producing a controlsignal representative of whether said amplitude exceeds a giventhreshold level in response to the amplitude of said well loggingsignal; selecting one of said at least two separate resolution signalsfor application to an indicating means in response to said controlsignal; producing a relatively high amplitude signal in response tochanges in said control signal; and combining said high amplitude signalwith said selected given function signal so that the combined signalwill have a large change in aMplitude level whenever said control signalchanges state.